As a conventional magnetic sensor (hereinafter referred to as an MI sensor) using a magneto-impedance element (hereinafter referred to as an MI element), for example, a magnetic sensor having a detection coil wound around an amorphous wire is known. Patent Document 1 discloses a MI sensor which causes a pulse current to pass through an amorphous wire and measures a first pulse of an induced voltage output from a detection coil to make it possible to sensitively detect an external magnetic field Hex. The MI element is also called a giant magneto-impedance element or a GMI element. The MI sensor is also called a giant magneto-impedance sensor or a GMI sensor.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2000-258517
As will be described below, a principle of magnetic field detection by the MI element will be explained with reference to FIG. 10.
As shown in the drawing, when a pulse current I passes through an amorphous magnetic wire 91, a magnetic field H is caused in a circumferential direction by the pulse current I. An induced voltage (dH/dt) is output from a detection coil 95. In the case where an external magnetic field Hx is applied in the state that the pulse current I flows, spins arranged in a circumferential direction of the amorphous magnetic wire 91 resonate and shake by θ. An induced voltage (dMθ/dt) generated by the spin resonance θ is output to the detection coil 95 overlapping the induced voltage (dH/dt). More specifically, in application of an external magnetic field Hx, an induced voltage (dH/dt+dMθ/dt) is output.
FIG. 11 is a waveform chart showing an output of an induced voltage to a pulse current of the MI sensor using the MI element. This is a waveform chart 101 showing a time variation of induced voltage in damped oscillation output from a detection coil when the pulse current I flows in application of the external magnetic field Hx.
FIG. 12 shows a waveform chart 102 showing a time variation in induced voltage caused only by the pulse current I without applying the external magnetic field Hx with respect to a peak characteristic of the first pulse in the waveform chart 101, a waveform chart 103 and a waveform chart 104, each showing a time variation in induced voltage when the external magnetic field Hx is applied (+Hx and −Hx).
As shown in FIG. 12, times (t) for zero crossing of the induced voltages when the first pulses damped in the waveform charts 102 to 104 are not equal to each other and have a phase difference. With respect to time t1 at which zero crossing is caused only by the pulse current I without applying an external magnetic field, time at which zero crossing occurs when an external magnetic field +Hx is applied is given by t1+Δta to cause a delay (Δta). When an external magnetic field −Hx is applied, the time for zero crossing is given by t1−Δtb, the zero crossing time is earlier (Δtb).
As a result, when the external magnetic field changes in polarity from +Hx to −Hx (FIG. 12), it is found that time at which an output voltage of the detection coil reaches a peak varies as zero crossing time varies. The present inventors devotedly studied the cause, and conceived the followings.
There is a phase difference in peak time between a time variation waveform of an output voltage by dH/dt which is a component caused by a pulse current and a time variation waveform of an output voltage by dMθ/dt which is a component varying (changing) depending on an external magnetic field. For this reason, an induced voltage waveform generated in the detection coil, which is a combination of two waveforms, has a phase difference with respect to peak time of an output voltage generated by dH/dt which is a component caused by the pulse current. A peak voltage of a time variation waveform in output voltage by dMθ/dt which is a component varying depending on the external magnetic field rises with an increase in external magnetic field. For this reason, a phase difference of an induced voltage waveform generated in the detection coil which is a combination of two waveforms is supposed to be changed with respect to peak time of an output voltage generated by dH/dt as the external magnetic field changes.
As will be described below, a known magneto-impedance sensor (hereinafter arbitrarily referred to as a MI element) uses the fact that a peak value of an output voltage of the detection coil is in proportion to an external magnetic field parallel to a magnetic sensitive member used in a magneto-impedance element (hereinafter arbitrarily referred to as a MI element).
In the MI sensor using the current MI element, on the basis of time t1 at which the pulse current rises as shown in FIG. 6 (described later), an analog switch is turned on-off for a short period of time at predetermined timing t2 at which a peak value is supposed to be given in an induced voltage waveform generated in the detection coil to detect a peak value of an output voltage generated in the detection coil corresponding to an external magnetic field. In this case, when a predetermined input current waveform and an output voltage waveform corresponding to the input current waveform are present, a sampling timeΔt is obtained by subtracting rise time t1 of the pulse current from a predetermined timing t2 at which a peak value is supposed to be given in the output voltage waveform (Δt=t2−t1).
In general, time at which an external magnetic field is applied such that a sampling time Δt is fixed in the absence of an external magnetic field (102 in FIG. 12) is also measured.
For this reason, when time at which an output voltage of the detection coil reaches a peak is varied by an external magnetic field as shown in FIG. 12, if the external magnetic field is applied, voltages are sampled at a timing deviated from time at which the output voltage reaches a peak value, so that sensitivity and linearity are deteriorated because the output voltage drops.
With a change in material characteristic of an electric resistance or the like of a magnetic sensitive member with a change in temperature, a pulse current flowing in the magnetic sensitive member changes. When the pulse current changes, a circumferential magnetic field H caused by the pulse current varies as a matter of course. Therefore, the circumferential magnetic field H caused by the pulse current independently of magnetization (M) of the magnetic sensitive member is varied by the change in temperature. According to this, deterioration in linearity and drift of an original point (in this example, a peak value of an output voltage generated without applying a magnetic field) occur.